It is known to use face sealing annular valves to regulate the flow of hydraulic fluid into or out of a piston cylinder, or other working chamber of a fluid-working machine. In annular valves of this type, a ring-shaped fluid path is selectively sealed by a similarly ring-shaped annular valve member. Annular valves are advantageous as they can provide a relatively large cross-sectional area through which hydraulic fluid can flow into or out of a working chamber.
Face sealing annular valves comprise an annular valve member which is typically mounted on a guide and they slide axially backward and forwards on the guide in use. However, a known problem with face sealing annular valves is that the annular valve members can jam in use if they tilt too far relative to the guide.
It is known to address this problem by providing annular valve members which are relatively long compared to their radius. If the length/radius ratio is sufficiently large, the annular valve member will not be able to tilt sufficiently far relative to the guide for the annular valve member to jam. However, it is undesirable for annular valve members to be long as this increases their mass and can increase the overall size of the annular valve.
The problem may also be addressed by using an annular valve member which can only move along a relatively short axial range and which has an inner diameter which is significantly larger than the outer diameter of the guide. In this case, jamming is unlikely, but the annular valve member can move significantly in a radial direction. Without sufficient restriction on radial motion, it is difficult to form an effective seal.
Therefore, the invention aims to provide an annular valve including an annular valve member which does not jam in use and which has restricted radial motion to enable a good seal to be formed. The invention also addresses the problems of keeping drag and friction forces low to allow for rapid and efficient operation and aims to provide an annular valve member with sufficient strength to withstand the stress of high pressure oil acting on it in use.